When designing an electronic circuit, it is sometimes an issue to recognize an active pulse in a periodical signal coming from other electronic module within the system. It is particular true when periodical signal concerned does not have an industrial standard definition with respect to the polarity of the active pulse in the periodical signal. For instance, with respect to VGA (video graphic adapter) subsystem of a personal computer, different makers defines active high Hsync, Vsync signals or active low Hsync, Vsync signals under different display modes respectively.
As illustrated in FIG. 1, a periodical signal 10 has an active high pulse 101 under one operation mode while the same periodical signal 20 has an active low pulse 201 under another operation mode. When a subsystem receives this kind of interface signal from another subsystem, the designer has to devise some way to overcome the compatibility issue.
The main object of the present invention is to provide a solution to generate active pulse of desired polarity from an ambiguous input periodical signal.